1. Field of the Invention
The present invention relates to an intake- and/or exhaust-valve timing control system which is optimally adapted for use in internal combustion engines, and specifically to a system which is variably capable of controlling the intake- and/or exhaust-valve timing depending upon the operating state of the engine, for example the magnitude of engine load and/or engine speed.
2. Description of the Prior Disclosure
Recently, there have been proposed and developed various intake- and/or exhaust-valve timing control systems for internal combustion engines for generating optimal engine performance depending on the operating state of the engine.
As is generally known, the valve timing is determined such that optimal engine performance is obtained, however the predetermined valve timing is not suitable under all operating conditions. For instance, when the engine is operating within a range of low engine rotational speeds, higher torque will be obtained with an intake-valve timing earlier than the predetermined valve timing.
Such a conventional intake- and/or exhaust-valve timing control system for internal combustion engines has been disclosed in Japanese Patent First Publication No. 1-300006 corresponding to German Patent Application No. P3810804.6. In this conventional valve timing control system, a cam sprocket having a driven connection with an engine crankshaft is rotatably supported through a ring gear mechanism at the front end of a camshaft. The ring gear mechanism includes a ring gear having an inner toothed portion engaging another toothed portion formed on the front end of the camshaft and an outer toothed portion engaging an inner toothed portion formed on the inner peripheral wall of the cam sprocket. In this manner, the ring gear rotatably engages between the cam sprocket and the camshaft. At least one of the two meshing pairs of gears is helical. The result is that axial sliding movement of the ring gear relative to the camshaft causes the camshaft to rotate about the cam sprocket and therefore the phase angle between the camshaft and the cam sprocket (and consequently, the phase angle between the camshaft and the crankshaft) is varied relatively. The ring gear is axially moved by the pressure difference between working fluid pressures applied to two pressure chambers, respectively defined at both ends of the ring gear in conjunction with the inner peripheral wall of the cam sprocket and the outer peripheral wall of the front end of the camshaft. A two-position spool valve is provided to supply fluid pressure from an oil pan through an engine oil pump to one pressure chamber defined in one side of the ring gear and in addition to exhaust fluid pressure from the other pressure chamber defined in another side of the ring gear to the engine oil pan. The former hydraulic circuit corresponds to an oil-supply hydraulic circuit, whereas the latter hydraulic circuit corresponds to an oil-exhaust hydraulic circuit. Both oil-supply and oil-exhaust hydraulic circuits are connected via the previously noted one spool valve to the pressure chambers. A spool slidably enclosed in the two-position spool valve is switchable by means of an electromagnetic actuator assembly attached to a rocker cover. The spool valve assembly and the electromagnetic actuator assembly are coaxially arranged with respect to each other. The plunger piston is directly connected to the spool so as to operate the spool valve between two positions.
In the aforementioned constructions, the conventional valve timing control system can provide a superior step-response and a relatively wide adjustable amount of the valve timing. However, since the electromagnetic actuator is disposed essentially in the vicinity of the front end of the camshaft, the entire length of the valve timing system is increased and as a result the overall engine size and engine weight become large. Therefore, the lay-out of the engine may be limited in the engine room.